Apparatus for ionizing particles in a mass spectrometer



' Jan. 10, 1967 G. HALL ETAL APPARATUS FOR IONIZING PARTICLES IN A MASS SPECTROMETER 3 Sheets-Sheet 1 Filed June 26, 1961 Jan. 10, 1967 L. G. HALL ETAL 3,297,894

APPARATUS FOR IONIZING PARTICLES IN A MASS SPECTROMETER Filed June 26, 1961 3 Sheets-Sheet z E 56 J o 60 v gZz ZWR Ar 5 ATmP/M/f Jan. 10, 1967 L. G. HALL ETAL APPARATUS FOR IONIZING PARTICLES IN A MASS SPECTROMETER Filed June 26. 1961 5 Sheets-Sheet 3 AZZ United States Patent 3,297,894 APPARATUS FOR IONIZING PARTICLES IN A MASS SPECTROMETER Lawrence G. Hall, West Covina, and Patrick F. Howden,

Los Angeles, Calif., assignors t0 SDS Data Systems, a

corporation of California Filed June 26, 1961, Ser. No. 119,676 7 Claims. (Cl. 313--63) This invention relates to apparatus for ionizing neutral particles and more particularly to an ion producing arrangement which is adaptable to use for ionizing particles in space.

Since it is ditficult to directly analyze neutral particles it is common in the field of particle analysis to first convert the particles into ions which may be more readily analyzed. Generally this is accomplished by capturing the particles in a vessel and subjecting them to an electron beam which passes in a straight line between a pair of spaced electrodes within the vessel. The electrons of the electron beam in passing between the electrodes collide with particles to form ions. The ions are then drawn from the vessel by an electric field for analysis in an ion analyzer such as a mass spectrometer.

The above described apparatus is generally employed in the laboratory and is useful in the analysis of most particles. However, there are particles, such as free radicals, which are difficult to analyze in the presence of confining structure which may affecttheir nature or create undesirable chemical reactions. Also, in practice, it has become desirable to provide apparatus for ionizing particles which does not interact with the particles being ionized. This is particularly true in the field of space mass spectrometry wherein it is desired to analyze particles appearing in free space around a moving satellite. Since the satellite is rotating and traveling at high velocity as it orbits the earth, particles appearing in free space stream past and onto the surface of the satellite. To ionize the space particles utilizing the prior art type of ionizing apparatus it would be necessary to project a pair of spaced electrodes beyond the surface of the satellite to create a straight line beam of electrons for ionizing particles between the electrodes. Due to the velocity and rotation of the satellite, the electrodes would disrupt the flow of space particles thereby presenting a disturbed sample of particles for ionization. Also, the particles upon being ionized might collide with the electrodes and be neutralized, thereby presenting an erroneous sample of ions for analysis to the ion analyzer apparatus enclosed Within the satellite.

In view of the above, the present invention provides means for ionizing particles without appreciable physical interference either with the particles or the ions formed from the particles.

To accomplish this, the present invention in a basic form includes apparatus isolated from the desired ionizing region of neutral particles. A beam of electrons removed from the ionizing region is projected to follow a curved path from the isolating structure into the ionizing region and caused to return to the isolating structure. The electrons in passing into and out of the ionizing region collide with particles to form ions. The ions are then drawn to receiving means located within the isolating structure for analysis.

More particularly, to project a beam of electronsin a curved path through the ionizing region, the present invention includes means for producing a curved magnetic field through the ionizing region and means for introducing electrons into the magnetic fieldthe electrons being constrained to follow the curved magnetic field to ionize the particles in the ionizing region.

To provide means for drawing the ions to the receiving 3,297,894 Patented Jan. 10, 1967 means located within the isolating structure, means are included for developing an electric field transverse to the magnetic fieldthe ions being drawn by the force of the electric field to the receiving means.

To improve the concentration of ions received by the receiving means, a lens arrangement is provided within the isolating structure. Preferably the outer structure of the lens is maintained at such a potential that an electric field is created adjacent to the lens which is greater than the electric field in the ionizing region. This creates a leakagefield into the ionizing region to produce a focusing action upon the ions. Thus an ion formed upon a collision between an electron and a particle is drawn from the ionizing region by the transverse electric field and focused by the lens arrangement to the receiving means which is positioned at the focal point of the lens.

The above as well as other features of the present invention may be more fully understood by a reference to the following detailed description when considered with the drawings, in which:

FIGURE 1- is a perspective representation of an embodiment of the ion producing apparatus of the present invention;

FIGURE 2 is a schematic representation of the apparatus illustrated in FIGURE 1;

FIGURE 3 is. a perspective representation of another embodiment of the ion producing apparatus of the present invention;

FIGURE 4 is aschematic representation of the apparatus illustrated in FIGURE 3; and

FIGURE 5 is a schematic representation of apparatus for ionizing particles particularly useful in a pressure gauge.

As briefly described above, the present invention includes means remote from an ionizing region including neutral particles, for developing a beam of electrons which is projected into the ionizing region and back to a point remote from the ionizing region and means for drawing ions formed by collisions between the electrons and the particles to a point remote from the ionizing region for analysis. To accomplish this, the embodiment of the present invention represented in FIGURES 1 and 2 includes an electron emission means in the form of a filament 10. The filament 10 is mounted upon a base 12 of magnetic material by a pair of rods, one of which is represented at 14. Preferablythe rod 14 is composed of a conductive material and is insulated from the base 12 by an insulating mounting represented at 15. To cause the filament 10 to emit electrons the filament 10 is coupled by its supporting rods through an emission regulator 16 to a power supply 18. Current from the power supply 18 flowing through the filament 10 causes the filament to heat up and emit electrons.

To accelerate the emitted electrons, as well as to control the projection of the electrons from the surface of the filament 10, an accelerator plate 20 is provided adjacent to the filament 10. The accelerator plate 20 includes an opening 22 through which the electrons emitted by the filament 10 may pass and is supported above the filament 10 by a pair of conductive rods, one of which is represented at 24. The rod is insulated from the base 12 by an insulating mounting represented at 26.

The accelerator plate 20 is maintained at a positive potential relative to the filament 10. This is accomplished by connecting the accelerator plate 20 to a movable contact arm 28 of a voltage divider arrangement 30. In practice, this connection may be made through the r-od.24.

The voltage divider arrangement 30 basically includes a resistance element 32, the terminals of which are connected to a source of positive and negative potential, represented generally at B+ and B, respectively; Positionable along the resistance element 30 are a plurality of accelerator plate.

movable contact arms 28, 34, and 36the contact arm 34 being coupled to the filament 10.

Due to the potential difference between the accelerator plate 20 and the filament 10, electrons are drawn from the vicinity of the filament and through the opening 22.

Under proper conditions, an electron accelerated along a magnetic field will follow or spiral along a line of fiux and will tend to follow the line of flux regardless of the curvature of the flux path. Accordingly, to cause the electrons emitted by the filament 10 to traverse a curved path through the ionizing region 38, the present invention includes means for generating a strong curved magnetic field through the ionizing region 38. In the embodiment represented in FIGURES 1 and 2, this is accomplished by providing bar magnets 40 and 42 which are spaced from each other and mounted on the base of magnetic material 12. The magnet 40 is positioned adjacent to the filament 10 with its south pole face facing the accelerator plate 20. The magnet 42 has its north pole face facing the accelerator plate 20. Accordingly, curved lines of magnetic flux are set up between the magnets 40 and 42. The curved path of the lines of magnetic flux as well as their density in the ionizing region may be controlled to some extent by a shaping of the pole faces adjacent to the For example, by making the pole faces of the magnets 40 and 42 substantially parallel to the accelerator plate lines of flux may be created'which approximate semicircular arcs in passing through theionizing region 38 between the magnets 40 and 42. Thus by positioning the filament 10 over a flux line which traverses a curved path between the magnets 40 and 42 electrons emitted by the filament 1t) follow, approximately, a semicircularly curved spiraling path through the ionizing region 38 toward the magnet 42. However, as illustrated in FIGURES 1 and 2, it has been found that in practice that in practice that it is preferable to inwardly shape the pole faces of the magnets 40 and 42 at an angle relative to the accelerator plate 20, such as forty-five degrees (45). This materially increases the density of the curved lines of magnetic flux in the ionizing region 38. Thus with the pole faces of the magnets shaped to make substantially a forty-five degree angle with the plane of the accelerator plate 20, electrons emitted by the filament 10 over the shaped portion of the magnet 40 more readily follow a curved spiraling path such as represented at 44 through the ionizing region 38 toward the magnet 42.

To collect the electrons emitted by the filament after passing through the ionizing region 38, an electron collector means 46 is provided. The electron collector means 46 preferably takes the form of a basket for catching the electrons. The collector means 46 is supported on the base 12 above the north pole face of the bar magnet 42 by a pair of conductive rods, one of which is represented at 48. Preferably to aid in the collection of electrons emitted by the filament 10 the collector 46 is positioned over the shaped portion of the north pole face of the magnet 42 and tilted along the direction of the curved lines of magnetic fiux between the magnets 40 and 42. The collector 46 is isolated from the ionizing region 38 by the accelerator plate 20 which is provided with a slot 50 above the collector 46 to allow electrons to pass to the collector 46. As represented by the battery 52, the collector 46 is maintained at a positive potential relative to the filament 10 and the accelerator plate 20 to aid in the collection of electrons. Accordingly, electrons traversing the curved path 44 through the ionizing region 38 are forced toward the accelerator plate 20 through the opening 50 to the electron collector 46.

As described above, electrons emitted by the filament 10 are initially accelerated and directed predominantly along the direction of the curved magnetic field, following which they are constrained to traverse a curved path substantially defined by the magnetic field to the electron collector means 46 through the ionizing region 38. This condition is readily achieved for a given physical configuration by is maintained at a positive potential so designing or adjusting the relative values of the magnetic field strength, electric field strength, and electron accelerating potential. In passing through the ionizing region 38, electrons collide with particles which, due to the collision, lose an electron and become positively charged ions. The rate at which ions are formed in the ionizing region 38 may be determined by several factors, including the density of neutral particles in the ionizing region and the number of electrons emitted by the filament 10.

To control the number of electrons emitted by the filament 10, the present invention includes an emission regulator 16. By way of example, the emission regulator 16 may be of the type described at page 144 of Modern Mass Spectrometry, 1953 edition, published by The Institute of Physics (British publication). As represented in FIGURE 2, electrons received by the collector means 46 are transported to the emission regulator 16 and compared with a reference signal from the power supply 18 to control the current flowing to the filament 10 at a predetermined value. The current flowing to the filament 10 in turn determines the rate of electron emission by the filament 10 and hence the rate of ionization of the particles appearing in the ionizing region 38.

Electron emission regulation is particularly useful in the field of mass spectrometry wherein it is desired to provide a stable reference for the number of ions flowing into the mass spectrometer for analysis.

As briefly mentioned above, the present invention includes means for drawing ions from the curved magnetic field within the ionizing region 38 to a point remote from the ionizing region 38. To accomplish this, the present invention provides apparatus for developing an electric field transverse to the curved magnetic field. Accordingly, ions formed in the ionizing region 38 are acted upon by the transverse electric field and are drawn from 'the ionizing region.

To provide a desired electric field, the embodiment of the invention represented in FIGURES 1 and 2 includes a metallic repeller grid structure represented generally at 54 which is supported above the base 12 by a pair of insulated rods 56 and 58, respectively. Due to the screen arrangement of the repeller grid 54, it is substantially transparent to uncharged particles. The repeller grid 54 by a direct connection to the source of positive potential B+. In practice, this may be accomplished by an electrical connection through one of the rods 56 or 58 which may be conductive. Due to the positive potential of the repeller grid 54, an electric field is produced which extends from the repeller grid 54 toward the accelerator plate 20. Thus, positive ions formed in the ionizing region 38 are drawn from the magnetic field toward a central opening 59 in the accelerator plate 20. If it is desired to draw in negative ions, the electric fields should be reversed.

To provide means for receiving'the ions which pass through the opening 59 that they may be transported to an ion collecting or analyzing arrangement, such as a mass spectrometer, the present invention includes ion receiving means positioned beneath the opening 59. The ion receiving means represented in FIGURES l and 2 preferably takes the form of a plate 60 having an opening 62 for receiving the ions. As represented in FIG- URE 1, the plate 60 is positioned in a plane beneath the magnet base 12 and provides an opening into an ion analyzer, represented generally at 64. The receiving plate 60 as well as the base 12 are maintained at a negative potential by a direct connection to the source of negative potential, B. Thus the ions drawn from the magnetic field by the electric field developed by the repeller grid 54 are attracted toward the receiving plate 60.

To improve the concentration of ions received at the receiving plate 60, a lens arrangement 66 is disposed between the ionizing region 38 and the receiving plate 60. The lens 66 is mounted above the base 12 between the magnets 40 and 42 by four insulated rods two of which are represented at 68 and 70. The lens 66 basically includes three focusing members 72, 74 and 76. The focusing members 72, 74 and 76 are aligned along a central axis to focus ions in the ionizing region 38 at the receiving plate 60 which may be placed at the focal point of the lens. The members 72 and 76 are electrically connected to the source of negative potential B- and the member 74 to the movable arm 36. Due to the electrical connection of the member 72 to B- a large electric field is developed between the outer lens member 72 and the accelerator plate 20. In particular, this electric field is substantially greater than the electric field between the repeller grid 54 and the accelerator plate 20. Accordingly, a leakage field is created into the ionizing region 38. The leakage field has the effect of producing a focusing of ions in the ionizing region 38 to the lens 66. Thus ions in the ionizing region 38. lying within the space defined by the broken lines78 and 80 traverse a converging curved path into the lens 66. The curved path aids the lens 66 in focusing the ions at the receiving plate 60. The ions lying outside the lines 78 and 80 in being focused by the lens 66 contact the accelerator plate 20, the focusing members of the lens 66 or the receiving plate 60 and are neutralized. Thus the portion of the ionizing region 38 lying within the lines 78 and 80 represents a preferred ionizing region from which ions are drawn to be analyzed by the ion analyzer 64.

Thus, electrons emitted by the filament traversing a curved path to the collector 46 collide with particles in the ionizing region 38 to form ions which, due to the electric field in the ionizing region, are drawn from the ionizing region to the lens 66 and focused at the opening 62 in the receiving plate 60 for analysis within the ion analyzer 64. To prevent the flow of ions through theion lens 66 to the receiving plate 60 from being affected by any electric fields caused by potentials on the supporting rods to the filament 10, the accelerator plate 20, collector 46 and/ or the lens 66, a shield represented at 82 is mounted on the receiving plate 60 surrounding the opening 62.

Although, as described above, it was considered that the electrons in traversing a path between the filament 10 and the collect-or 46 traveled a path which substantially conformed to the curved magnetic field between the bar magnets 40 and 42, due to the crossing of the magnetic and electric fields, the electrons travel a slightly perturbed path to the collector 46. As an electron is drawn from the filament 10 and is accelerated along a magnetic line of flux between the magnets 40 and 42, it is acted upon by the electric field which tends to draw the electron toward the repeller grid 54. The electron is being drawn toward the repeller plate 54 crosses magnetic lines of flux which tends to cause the electron to move in a circular path normal to the magnetic field. As a result of the forces of the crossed fields the electron is caused to traverse a repetitive cycloidal path with reference to the curved lines of magnetic flux extending between the bar magnets 40 and 42. The average motion of the electrons relative to a plane extending between the magnets 40 and 42 forms an angle from the filament 10 the tangent of which is proportional to a magnitude ratio of the electric and magnetic fields. Accordingly, in the embodiment of the present invention represented in FIGURES l and 2, electrons emitted by the filament 10 traverse a curved path in a plane which makes a small angle relative to the center plane extending between the bar magnets 40 and 42. Thus the collector 46 must be displaced by a small amount from the plane of the magnets 40 and 42, the

amount of displacement of the collector means 46 depending upon the magnitude of the magnetic and electric fields as well as the distance between the bar magnets 40 and 42 and the initial velocity imparted to the electrons.

As described above, the electric field tends to attract the electrons emitted by the filament 10 toward the repeller grid 54. Thus the magnitude of the electric field developed in the ionizing region relative to the magnitude of the magnetic field must be controlled to prevent undesired escapage of electrons from the magnetic field. For example, it has been found that when the electrons emitted by the filament 10 are subjected to an accelerating potential of tens to hundreds of volts, that an electric field (E) of about 250 volts per centimeter between the repeller grid 54 and the accelerator plate 20 and a magnetic field (B) of about 200 gauss in the ionizing region 38 provides a suitable electron ionizing beam configuration.

As represented in FIGURE 1, in a practical embodiment of the present invention, the apparatus for producing the curved magnetic field as well as the emission of electrons to follow a curved path between the filament 10 and the collector 46 is isolated from the ionizing region 38 by a casing 84. The casing 84 includes slots which communicate with the openings in the accelerating plate 20. Accordingly, the electrons are free to pass from the filament 10 to the collector 46 and ions are free to pass through the lens 64 to the opening 62 in the receiving means 60.

The isolation of the apparatus of the present invention from the ionizing region 38 avoids considerable undesired disturbance of the particles within the ionizing region 38 as well as of the ions formed by collision between the particles and the electrons emitted by the apparatus of the presnet invention. As previously mentioned, this is of prime importance in the field of space mass spectrometry. Thus the apparatus of the present invention may be mounted within the satellite, a surface of which with suitable openings therein is depicted by way of illustration at 86. As represented at 87 in FIGURE 1, in this arrangement the accelerator plate 20 and the surface 86 are electrically coupled to each other. This prevents undesired electric fields from existing between the accelerator plate 20 and the satellite which might effect paths traversed by the electrons and ions between the surface 86 and the accelerator plate 20. Thus, with the present.

invention positioned within the satellite a means is provided for ionizing the space particles which stream past and into the satellite without appreciably interfering with the stream of particles or with the ions produced by operation of the present invention.

To further reduce any interference with the particle stream, it may be desired to eliminate entirely any structure remote from the surface of the satellite or extending beyond the casing 84. Thus it may be desired to eliminate the repeller grid 54- and the supporting rods 56 and 58. In this case, the potential of the accelerator plate 20 relative to lens element 72 would necessarily be increased to provide a leakage electrical field transverse to the magnetic field sufiicient to draw ions from the ionizing region 38 through the lens 64 to the receiving plate 60. Also, in general, the potential of the accelerator plate 20 relative to the filament 10 would be increased.

Referring to FIGURES 3, 4 and 5 there is shown an embodiment of the present invention which has the advantage of not necessitating a critical positioning of the electron collector means relative to the electron emission means due to the path traversed by electrons emitted by the electron emission means in crossing magnetic and electric fields. This embodiment is also particularly useful as ionizing apparatus for a pressure gauge arrange ment.

As represented, to provide a source of electrons, a filament 88 is mounted by means of a pair of conductive rods, one of which is shown at 90, on a base of magnetic material 92. The rod 90 is insulated from the base 92 by aninsulating mounting represented at 94.

To cause the filament 88 to emit electrons the filament 88 is coupled by its supporting rods-through an emission regulator 96 to a power supply 98. Current from the power supply 98 flowing through the filament 88 causes the filament to heat up and emit electrons.

The filament 88 is also maintained at a positive potential from a voltage divider arrangement, indicated generally at 100 in FIGURE 4. The voltage divider arrangement 100 includes a. resistance element 102, the terminals of which are connected to a source of positive potential and a source of negative potential represented at B-land B, respectively. Disposed along the resistance element 102 are a plurality of movable contact arms 104, 106, and 108, the filament 88 being connected to the movable arm 108.

To accelerate the emission of electrons from the filament 88, an accelerating plate 110 having an opening 112 for passing electrons is mounted by a pair of insulated rods 114 and 116 on one side of the filament 88. As represented in FIGURE 4, the accelerating plate 110 is maintained at a positive potential relative to the filament 88 by a connection to the contact 106. Accordingly, electrons emitted by the filament 88 are accelerated through the opening 112 into an ionizing region, represented generally at 118.

In accordance with the present invention, the electrons emitted by the filament 88 are caused to traverse a curved path through the ionizing region 118 to an electron collector means which is positioned remote from the ionizing region 118. In particular, in the embodiment of the pressent invention represented in FIGURES 3, 4 and 5, the collector means includes a ring-shaped collector plate 120 radially spaced from the filament 88 and the accelerating plate 110. The collector plate 120 is mounted on supporting base 122 by a pair of rods, one of which is represented at 124. As represented in FIGURES 4 and the collector plate 120 is maintained at a positive potential relative to the filament 88 by a connection to the emission regulator 96 through a battery 134. In practice this electrical connection may be through the rod 124 which is conductive and insulated from the supporting base 118 by an insulating mounting 126.

To provide means for causing the electrons emitted by the filament 88 to traverse a curved path through the ionizing region 118 to the collector plate 120, the present invention includes means for generating a curved magnetic field through the ionizing region 118. In the embodiment of the present invention illustrated in FIGURES 3, 4, and 5 this is accomplished by a bar magnet 128 surrounded by a ring-shaped magnet 130. The bar magnet 128 is mounted on the base of magnetic material 92 adjacent to the filament 88, and has one of its pole faces (the north pole) positioned on one side of the plane of the collector plate 120 facing the filament 88. The ringshaped magnet 130 is mounted on the base 92 adjacent to the ring-shaped collector plate 120 and has one of its pole faces (the south pole) facing the collector plate 120 on the same side of the plane of the collector plate 120 as the bar magnet 128. Accordingly, curved lines of magnetic flux are set up between the bar magnet 128 and the ring-shaped magnet 130 and completed through the base 92. As described similarly in connection with FIG- URES l and 2, electrons, emitted by the filament 88 and initially accelerated by accelerating plate 110 along the direction of the curved lines of magnetic flux extending between the magnets 128 and 130, traverse a spiraling curved path to the collector plate 120 through the ionizing region 118. In traversing the curved path, which is represented generally by the dotted line 132, the electrons collide with particles to form ions. The rate at which ions are formed in the ionizing region 118 may be determined by several factors, including the density of neutral particles in the ionizing region and the number of electrons emitted by the filament 88.

To control the number of electrons emitted by the filament 88, the present invention includes an emission regulator 96 which may be of the type described in connection with FIGURE 2. Thus electrons received by the collector plate 120 are transported through a battery 134 to the emission regulator 96 and compared with a reference signal from the power supply 98 to control the current flowing to the filament 88 at a predetermined value. The current flowing to the filament 88 in turn determines the number of electrons emitted by the filament 88 and hence the rate of ionization of the particles appearing in the ionizing region 118.

To provide means for drawing the ions from the ionizing region 118 that they may be analyzed, the present invention includes means for developing an electric field transverse to the curved magnetic field. This may be accomplished by a repeller grid 140 mounted by a pair of insulated rods 142 and 144 which is substantially parallel to and remote from the plane of the collector plate 120. Due to the screen-like structure of the repeller grid 140, it is substantially transparent to neutral particles passing into the ionizing region 118. The repeller grid 140 is maintained at a positive potential relative to the accelerating plate and the electron collector plate by a direct connection to the movable contact arm 104. In practice this may be accomplished by a connection through one of the rods 142 or 144 which may be conductive. Since the base 92 is maintained at a relative negative potential by a direct connection to the source of negative potential B-, electric fields are set up between the repeller grid and the accelerating plate 110 and between the accelerating plate 110 and the base 92 which are transverse to the curved magnetic field. Thus ions formed by collisions of electrons with particles located in the ionizing region 118 are drawn from the magnetic field towardthe base 92.

To provide means for collecting the ions, a receiving plate 146 is disposed in the annular slot 148 between the bar magnet 128 and the ring-shaped magnet 130. Thus, electrons emitted by the filament 88 and traversing a curved path through the ionizing region 118 to the collector plate 120 collide with particles and form ions which are drawn from the ionizing region by the electric field between the repeller grid 140 and the base 92 to the ion receiving plate 146 along a path indicated by the broken line 150. The ions may then be passed to an ion analyzer for analysis as indicated by the flow line 151.

To maintain the path of ion fiow unaffected by an electric field which may be caused by potentials on the supporting rods of the filament 88 and the accelerating plate 110, an insulating shield 152 is mounted on the base 92 around the magnet 128.

As described in connection with FIGURES 1 and 2,

due to the crossing electric and magnetic fields, electrons tron collector 120 is a circular plate and since the electrons are emitted from a point centrally located in the plane of the collector 120 to traverse a curved path to the circular collector plate.

As mentioned above, the apparatus illustrated in FIG- URE 3 is particularly useful in providing ionizing apparatus for a pressure gauge which does not interfere with the particles, the pressure of which is to be determined.

To provide such an arrangement, the apparatus of FIG- URE 3 is modified in the manner represented in FIGURE 5. In particular, an accelerating electron collector plate 154 having an opening 156 is positioned within the opening 112 and maintained at a positive potential relative to the filament 88 by connection to a battery 138. Preferably plate 154 has an annular thickness which is substantially less than that of the plate 110. The ring-shaped collector plate and the accelerator plate 110 are maintained at a potential which is slightly negative relative to the filament 88 by a battery 158 and act as electron repeller plates. Thus electrons emitted by the filament 88 are accelerated by the plate 154 through the opening 156 into the ionizing region 118. Due to the curved magnetic field, the electrons are constrained to travel a curved path represented by the dotted line 160 toward the ringshaped plate 120. However since the plate 120 is negative relative to the filament 88 a portion of the electrons are reflected from the plate 120. These electrons are attracted toward the repeller grid 140 and constrained by the curved magnetic field to traverse a curved path substantially as represented by the dotted line 162 toward the filament 88. In entering the area adjacent to the fila ment 88 the electrons encounter'the relatively negative field of the plate 110. Since the plate 110 has an annular thickness which is substantially greater than that of the plate 154, only a small portion of the electrons are collected by the plate 154 while a major portion of electrons are again repelled back toward the plate 120 along a path substantially as represented by the dotted line 164. This, in effect, creates a cloud of electrons between the filament 88 and the collector plate 120 which collide with particles in the ionizing region 118. The collisions of the electrons with the particles produces ions, the number of ions being proportional to the density of the particles in the ionizing region and hence the pressure of the ionizing region. Accordingly, the number of ions collected by the ion collector plate 146 represents a measure of the pressure of the ionizing region 118. To determine the pressure of the ionizing region, the ions collected by the ion collector plate 146 are passed to an ion amplifier 166 and hence to a meter 168 which may be calibrated to directly measure ion flow in terms of pressure.

Although the structure shown in FIGURE 3 when modified in accordance with FIGURE has been described as being useful in ionizing particles for a pressure gauge, it is to be understood that the apparatus is also applicable to a mass spectrometer. In particular, in such a combination, a lens arrangement, similar to that described in connection with FIGURES 1 and 2, may be disposed within the annular opening 148 between the bar magnet 128 and the ring-shaped magnet 130 to focus ions on an opening in the mass spectrometer.

The apparatus described in connection with FIGURES 3 and 4 may also be useful in space mass spectrometry in that an electron beam may be directed from within isolating structure to an ionizing region and return to the isolating structure to ionize particles without interfering with the ionizing region or the ions formed by collisions of the electrons with the particles.

In a configuration wherein the apparatus of FIGURES 3 and 4 is utilized to provide means for ionizing particles in space, it may be desired to eliminate repeller grid 140. In such a case, the accelerating plate 110 and collector plate 120 may be maintained at larger positive potentials relative to the electron emission means, the magnets and shielding structure thereby providing a leakage electric field which is transverse to the curved magnetic field for drawing ions from the magnetic field to the ion receiving means.

What is claimed is:

1. Apparatus for ionizing particles in an ionizing region comprising: an electron emission means remote from the ionizing region; electron collector means spaced from the electron emission means and remote from the ionizing region; means for developing a magnetic field having lines of flux extending along a curve intersecting the electron emission means and the electron collector means through the ionizing region; means for accelerating electrons from the electron emission means in the direction of said flux lines where they intersect the emission means, whereby electrons emitted by the electron emission means travel a curved path to the electron collector means thereby converting particles in the ionizing region into ions; and

means for developing an electric field across the magnetic field to draw ions from the ionizing region.

2. The apparatus of claim 1 including means for focusing the ions drawn from the ionizing region at an ion receiving means.

3. Apparatus for ionizing particles in an ionizing region comprising: an electron emission means remote from the ionizing region; electron collector means spaced from the electron emission means and remote from the ionizing region; means for developing a magnetic field having lines of flux intersecting the electron emission means and the electron collector means along a curved path through the ionizing region; means for accelerating electrons emitted by the electron emission means in the direction of said flux lines where they intersect the electron emission means to follow the curved magnetic field through the ionizing region to the electron collector means thereby converting particles in the ionizing region into ions; means for developing an electric field across the magnetic field to draw the ions from the ionizing region to an ion receiving means; and means coupled to the electron emission means for controlling the rate at which electrons are emitted from the electron emission means to control the rate of ionization.

4. Apparatus for ionizing particles in an ionizing region comprising: an electron emission means remote from the ionizing region; electron collector means spaced from the electron emission means remote from the ionizing region; an electron accelerating plate positioned in a plane between the ionizing region and the electron emission means and having slots therein to provide means whereby electrons emitted by the electron emission means may pass into the ionizing region and return to an electron collector means spaced from the electron emission means and positioned to a side of the accelerator plate remote from the ionizing region; magnet means having a first one of its pole faces positioned adjacent to the electron emission means remote from the accelerator plate and tapered away from the plane of the accelerator plate toward the collector means; and a second one of its pole faces positioned adjacent to the collector means remote from the accelerator plate and tapered away from the plane of the accelerator plate toward the first magnet to produce a curved magnetic field between the first and second pole faces through the ionizing region whereby electrons emitted by the electron emission means follow a curved path to the electron collector means to ionize particles in the ionizing region.

5. The apparatus defined in claim 4 wherein the pole faces of the first and second magnets are tapered away from the accelerator plate to make an angle of substantially 45 degrees with the accelerator plate.

6. Apparatus for ionizing particles in an ionizing region comprising: an electron emission means remote from the ionizing region; an electron collector means spaced from the electron emission means and positioned remote from the ionizing region; an electron accelerator means positioned adjacent the electron emission means in a plane between the ionizing region and the electron emission means; a first magnet having one of its pole faces positioned adjacent to electron emission means remote from the electron accelerator means and tapered away from a plane extending between the electron emission means and the electron collector means; a second magnet having one of its pole faces positioned adjacent the electron collector means remote from the electron accelerator means and tapered away from the plane between the electron collector means and the electron emission means toward the first magnet to produce a curved magnetic field between the first and second magnets through the ionizing region whereby electrons emitted by the electron emission means follow a substantially curved path through the ionizing region to the electron collector means to ionize particles in the ionizing region; means for developing an electric field across the curved magnetic field to draw ions from the ionizing region; and means for receiving ions drawn from the ionizing region positioned remote from the ionizing region.

7. Apparatus for ionizing particles in an ionizing region comprising: an electron emission means remote from the ionizing region; an electron accelerating plate positioned in a plane between the ionizing region and the electron emission means and having slots therein to provide means whereby electrons emitted by the electron emission means may pass into the ionizing region and return to an electron collector means spaced from the electron emission means and positioned to a side of the accelerator plate remote from the ionizing region; a magnet means having a first one of its pole faces positioned adjacent to the electron emission means remote from the accelerator plate and a second one of its pole faces positioned adjacent to the collector means remote from the accelerator plate to produce a curved magnetic field between the first and second pole faces through the ionizing region whereby electrons emitted by the electron emission means follow a curved path to the electron collector means to ionize particles in the ionizing region.

References Cited by the Examiner UNITED STATES PATENTS 2,935,634 5/1960 Lerbs 31363 JOHN W. HUCKERT, Primary Examiner.

ARTHUR GAUSS, DAVID J. GALVIN, Examiners.

R. F. POLISSACK, J. D. KALLAM, A ssislam Examiners. 

1. APPARATUS FOR IONIZING PARTICLES IN AN IONIZING REGION COMPRISING: AN ELECTRON EMISSION MEANS REMOTE FROM THE IONIZING REGION; ELECTRON COLLECTOR MEANS SPACED FROM THE ELECTRON EMISSION MEANS AND REMOTE FROM THE IONIZING REGION; MEANS FOR DEVELOPING A MAGNETIC FIELD HAVING LINES OF FLUX EXTENDING ALONG A CURVE INTERSECTING THE ELECTRON EMISSION MEANS AND THE ELECTRON COLLECTOR MEANS THROUGH THE IONIZING REGION; MEANS FOR ACCLERATING ELECTRONS FROM THE ELECTRON EMISSION MEANS IN THE DIRECTION OF SAID FLUX LINES WHERE THEY INTERSECT THE EMISSION MEANS, WHEREBY ELECTRONS EMITTED BY THE ELECTRON EMISSION MEANS TRAVEL A CURVED PATH TO THE ELECTRON COLLECTOR MEANS THEREBY CONVERTING PARTICLES IN THE IONIZING REGION INTO IONS; AND MEANS FOR DEVELOPING AN ELECTRIC FIELD ACROSS THE MAGNETIC FIELD TO DRAW IONS FROM THE IONIZING REGION. 